Since the passage of the Clean Water Act in 1972 the federal government through the Environmental Protection Agency (EPA) has mandated progressively tighter controls over the quantities of pollutants and contaminants that are allowed to be released into our nation's waters. These progressively tighter mandates also encompass control of peak flows and/or volumes and the rate at which they can be discharged into existing water ways or drainage infrastructures. These resulting mandates and controls have given birth to new programs and procedures collectively referred to as storm water management.
Devices, systems and procedures that remove or reduce the pollutants and contaminates and/or control peak flows and volumes are often referred to as best management practices (BMPs). The BMPs can utilize natural, artificial or man-made techniques, and even combinations of either and/or both. Some examples of these BMPs include trash filters, sedimentation basins, retention and detention ponds, wetlands, infiltration trenches, grass swales, various types of media filters, and various types of natural filter systems including sand filters, and aggregate filters including natural and artificial wetlands.
BMPs typically use one or more mechanisms to remove the pollutants and contaminates. These mechanisms include sedimentation, filtration, absorption, adsorption, flocculation, stripping, leaching, bioremediation, and chemical processes including oxidation reduction, ion exchange, and precipitation.
Common pollutants of concern typically found in stormwater flow are sediment, foliage, litter, nutrients, metals, hydrocarbons, and bacteria. Because pollutants of concern vary significantly in their respective physical characteristics, a variety of techniques are applied to better address difficulties with capturing each pollutant of concern. For example; capturing hydrocarbons, which are typically in a liquid state, will require a different technique than capturing litter which are in a solid state.
Furthermore, storm water treatment systems can also be classified in relationship to the treatment level in which they are being used. In this respect, the term treatment is generally used to describe the unit processes that are used to reduce the quantities of pollutants and containments in storm water runoff.
For example, basic or pre-treatment typically refers to the removal of gross solids, sediments and larger debris through the processes of settling and screening. Enhanced or advanced treatment typically refers to processes or reducing targeted pollutants.
Filtration is a common form of enhanced treatment for storm water. Filtration utilizes a combination of physical, chemical, and biological processes. Types of filtration greatly vary dependent on the media use. Medias can be both inert and/or sorbent and are also strongly linked to natural biological processes that thrive in and/or around the media environment. Advanced filtration techniques especially include chemical and biological processes and generally include, but are not limited to processes that bring storm water in contact with sorbent surfaces and the micro-organisms that colonize these surfaces. Reducing the pollutant loadings to a receiving body of water can avoid diminishing the ability of the receiving body of water to counter the pollutant load through natural processes present within the receiving body of water. Every water body has a maximum pollutant load that it can withstand without the receiving body of water going into decline.
Although media filtration of stormwater can be effective for pollution removal, contact time with the media is an important factor with the projected removal efficiency of the media. Greater contact time will achieve a higher removal efficiency. Creating water detention upstream from a media filter will enable the water flow through a filter to be slowed down in order to achieve greater water contact time with the media.
Stormwater treatment systems that make use of filtration media can vary significantly just by making use of different types of filtration media. Understanding that the effectiveness will vary for different types of filtration media, generally, filtration media functions by combining the processes of physical filtration, sorbent or reactive surfaces, and biological activity. Physical filtration will physically prevent the conveyance of solids, sorbent or reactive surfaces will chemically bond to the pollutants, and the biological activity will consume pollutants.
Physical filtration whether by use of a filtration media or a screen can be an effective technique for capturing solids. Contained within and attached to the solids are a wide spectrum of pollutants of concern that can range from nutrients, to litter, to bacteria. A difficulty with physical filtration is that the collected solids can become so numerous that the rate of filtration is slowed to an ineffective flow rate. Servicing of the filtration media is typically required, and the servicing often requires the media to be replaced as a part of servicing.
Another issue with managing filtration media is how to do the servicing and replacement of the media as the media wears out or become saturated over time. A critical element for any stormwater treatment system is being able to service the system quickly and easily. The longer it takes to service a stormwater treatment system, the more money it will cost for both the manpower and service equipment. In addition, there is an element of servicing that is centered around the safety of the service technicians.
For a service technician to enter a stormwater treatment vault, OSHA requires the service technicians to adhere to a protocol referred to as the confined space protocol. The protocol requires the service technician that enters the vault to be equipped with a significant amount of specialized equipment. The confined space protocol also requires more personnel to be involved in the process, and a detailed report that a confined space entry took place must be created and submitted to an administrator. If the service technicians can complete the servicing without having to enter the vault, the additional manpower and time spent can be avoided. It is important to be able to service filtration media quick and easy without requiring a service technician to enter the confined space of a vault.
Applying polymers to stormwater flow is a type of stormwater treatment that can be adapted to high flows. Polymers applied to water purification techniques have been used for over 75 years. Common applications for polymers include drinking water purification, erosion control, sanitary sewer processing, dredging, lake water quality enhancement, and more.
Polymers work in the following way: Solids that are sufficiently small do not settle easily and have a tendency to maintain suspension in the water column. These solids typically have a negative charge which enables them to repel from each other, and they are kept in motion by the phenomenon known as Brownian motion. Brownian motion maintains the suspension of these particles in the water column, relatively equally diffused throughout the water. These particles often carry a wide spectrum of pollutants of concern. Polymers act on these tiny particles to neutralize their charge. Once the charge on these particles have been neutralized, random motion of the water will direct the particles to bump into each other and cohesion will coagulate these small particles to form larger particles.
As the particles increase in size the diffusion energy that previously enabled Brownian motion becomes insufficient to maintain suspension of these particles in the water column. As a result, these larger particles will settle out of the water column and prevented from being transported to a receiving body of water. In addition, an unused portion of the polymer application will find its way into the receiving downstream water body where it will act to enhance the water quality there.
There are a couple of common problems with using polymers for stormwater applications. If the stormwater is overdosed with polymers the water can become toxic and affect a receiving body of water in a negative way. Overdosing is a concern when applying liquid polymers. Liquid polymers require significant human oversight to avoid overdosing a stormwater flow which adds significant costs to liquid applications of polymers. To overcome the need for human oversight of polymer dosing, polymers have been engineered into solid logs that dissolve in flowing water. These logs will not dissolve unless water is flowing across the surface of the logs. These surfaces of the polymer logs erode from the water flow, and as the logs continue to erode, polymers are released into the water flow. Dosing concentration is a factor of water flow rate and the number of logs engaged.
A problem with polymer logs is that they can become covered by solids and blinded so that the water flow does not erode the surface of the log. Grass and leaves are especially likely to quickly blind a polymer log, and most stormwater drain pipes commonly convey large quantities of grass and leaves.
Another issue with managing polymer logs is servicing and replacing the logs as they erode away over time. A critical element for any stormwater treatment system is being able to service the system quickly and easily. The longer it takes to service a stormwater treatment system, the more money it will cost for both the manpower and service equipment. In addition, there is an element of servicing that is centered around the safety of the service technicians. For a service technician to enter a stormwater treatment vault, OSHA requires the service technicians to adhere to a protocol referred to as the confined space protocol. The protocol requires the service technician that enters the vault to be equipped with a significant amount of specialized equipment.
A confined space protocol also requires more personnel to be involved in the process, and a detailed report that a confined space entry took place must be created and submitted to an administrator. If the service technicians can complete the servicing without having to enter the vault, the additional manpower and time spent can be avoided. It is important to be able to service polymer logs quickly and easily without requiring a service technician to enter the confined space of a vault.
The reduction of nutrients that are conveyed via stormwater runoff are in the forefront of the various pollutants of concern. For example; the EPA has mandated that the state of Florida reduce the overall pollutant discharge of the state by approximately ⅓. To this end the EPA has established a numeric nutrient criteria for all water bodies of the state for total phosphorus (TP) and total nitrogen (TN). These nutrient numeric values cannot be exceeded within these water bodies of the state. The result of exceeding these numeric values will result with the state of Florida being fined by the EPA.
The reduction of metals that are conveyed via stormwater runoff is also in the forefront of pollutants of concern. Mining can release large quantities of dissolved metals into stormwater runoff which eventually flows into streams and rivers with contain wildlife. Salman are extremely sensitive to metals in stormwater because they have delicate organs that are in continuous contact with the water in which the swim. Even low levels of dissolved metals can cause life threatening physiological functions. In addition, low levels of metals will bio-accumulate with in the tissues of a salmon concentrating the metals. In turn, other wildlife and humans will consume salmon along with the toxic metals that have accumulated and concentrated within the salmon's tissues. The EPA is on the forefront for imposing rules and guidelines to prevent metals pollution within United States water bodies.
An underlying problem with stormwater treatment is that detention of stormwater to achieve a greater pollutant removal efficiency reduces the hydraulic conveyance which may compromise the hydrology of the water shed. Historically, stormwater management has been primarily about flood prevention. Because of both governmental mandates and environmental necessity, today stormwater management includes the prevention of the conveyance of pollutants. There is a necessity to prevent both flooding and the conveyance of pollutants. Because there is not a single stormwater treatment technique that is the best for removing all pollutants of concern, a treatment system that combines multiple techniques in a treatment train arrangement will likely yield a wide spectrum of stormwater treatment success.
In addition, there is an economic consideration for applying stormwater treatment. For example; the costs and frequency for servicing a stormwater treatment system will be an indicator as to the sustainability of the system . . . all stormwater treatment systems require servicing.
Thus, the need exists for solutions to the above problems with the prior art.